1. Field of the Invention
The present invention relates to the direct reduction of iron oxides to elemental iron. More specifically, it relates to direct iron reduction processes utilizing one or more solid carbon reducing agents and a rotary hearth furnace to achieve continuous direct iron reduction.
2. Discussion of Related Art
Skilled artisans in the field of refining iron are increasingly recognizing direct reduction, which involves a chemical reduction reaction at a temperature below the melting temperature of the materials present, as a useful method of converting iron oxides, such as, for example, iron ore, into elemental iron. The two general categories of direct reduction are (1) those that utilize natural gas as the reducing agent, and (2) those that utilize solid carbonaceous materials such as coal as the reducing agent (solid-based direct iron reduction). While solid-based direct iron reduction is presently being given a great deal of attention as a potentially useful reduction mechanism, it is hindered by several efficiency problems. The present invention overcomes many of these problems by providing advantageous processes for achieving continuous direct iron reduction using one or more solid carbon reducing agents and a rotary hearth furnace.
Of all direct reduced iron ("DRI") currently being produced, at least 90% worldwide is produced using natural gas-based processes rather than solid carbon-based processes. These processes which utilize natural gas as the reductant typically involve expensive oxide pellets or lump ore as feed stock. It is believed that the only DRI currently being produced in any significant amount using solids-based processes is produced by the SL/RN or GRATECAR processes, which use oxide pellets or lump ore together with sized coal as the feed material. In these processes, there is no admixing of ground iron ore particles with a finely-divided particulate solid reductant. Rather, the large coal and ore lumps or pellets (typically having sizes of about 1/8 inch to about 2 inches) are fed into a rotary kiln furnace, and the reduction reaction proceeds very slowly, commonly requiring kiln residence times of over 12 hours. Consequently, for a large kiln, the equipment only produces about 50,000 to about 150,000 tons of DRI product per year, and this production involves very large capital costs.
There are currently under development three alternative processes for DRI production using coal as the reductant which utilize a rotary hearth furnace. In these rotary hearth processes for reducing iron ore by carbonaceous direct reduction, ground iron ore and a finely-divided carbonaceous reducing agent, as well as other additives, such as binding agents, are first formed into spherical agglomerates called green balls or into briquettes. One such process, termed the "FASTMET" process herein, involves producing large green balls (nominally 20 mm in diameter) of iron oxide and coal, and utilizes a thermal drier prior to introduction of the balls onto a rotary hearth furnace. The second process, termed the "INMETCO" process herein, is similar to the FASTMET process, but utilizes smaller green balls (nominally 9 to 12 mm in diameter), and does not use a drier. Instead, the green balls are fed "wet" directly onto the rotary hearth furnace. The third process, termed the "MAUMEE" process herein, is similar to the INMETCO process, but utilizes wet or dry briquettes formed under high pressure with or without binder material. The MAUMEE process uses very expensive briquetting machines to form dry or nearly dry briquettes that, depending upon the starting material, may or may not require one or more binders to achieve briquettes strong enough to withstand handling steps onto the hearth of a rotary furnace.
A major focus of developmental work in the field of carbonaceous direct reduction of iron, therefore, is the development of agglomerates having adequate tensile strength and abrasion resistance to prevent powdering or breaking of the agglomerates during conveyance to the rotary hearth. As such, both the FASTMET process and the INMETCO process, as well as other similar processes, have the inherent problem of requiring extremely large dosages of binders. The binders are required in such large proportions in order to make a wet or dry agglomerate of adequate strength and durability to avoid breakage during handling ahead of the drier or rotary hearth furnace, and to avoid exfoliation or explosion in the case of the INMETCO process where wet balls are introduced directly onto the hot hearth of the rotary hearth furnace.
These agglomerates, typically green balls but in some processes briquettes, are charged into a rotary hearth furnace, where the iron ore in the agglomerate is reduced to yield "sponge iron." The term "sponge iron" refers to the product of a direct reduction process and is used interchangeably herein with the term "DRI". The sponge iron, which is still in agglomerate form, normally is then densified by briquetting, shipped and melted to extract the reduced elemental iron from contaminants such as silica and sulfur, which are tightly bound to the elemental iron in the sponge iron product.
All three processes, the FASTMET process, the INMETCO process and the MAUMEE process are accompanied by very large capital and operating costs, which are associated in part with the necessity of binder materials and/or expensive equipment, and processing steps to form the iron ore and other starting materials into agglomerates. For example, the iron ore typically must be ground to a fairly stringent size specification before agglomeration. Additionally, the balling or briquetting step requires large capital costs due to the need for proper machinery for forming and sizing the agglomerates and adding the large amounts of binders needed to form balls or briquettes. Also, the agglomeration step typically requires the addition of moisture to the starting materials. Thus, in the FASTMET process, it is necessary to perform a drying step prior to charging agglomerates into a furnace for direct reduction. This drying step also requires additional machinery as well as large inputs of energy, and it also places greater demands on the strength and durability of the dry, and typically more fragile, green balls so that the dry ball arrives on the reduction hearth largely intact. Therefore, a significant amount of binder is required in the FASTMET process.
One manner of alleviating this problem is to grind the ore and the reductant to a much finer particle size distribution. For example, an agglomerate made for this process using course starting materials (i.e., where about 80% of the particles are less than about 100 .mu.m in diameter), the mixture to be formed into the agglomerate must typically include from about 0.5 to about 0.6% binder by weight (typically about half organic binders and about half bentonite clay). If, however, relatively fine starting materials are to be used (i.e., where about 80% of the particles are less than about 44 .mu.m in diameter), the process requires a somewhat smaller proportion of binder. However, grinding itself is associated with added costs, and only reduces the problem, but does not eliminate it.
In the INMETCO process, described above, moist pellets are placed directly into a rotary hearth furnace; however, in this type of process, there are several precautionary measures which must be taken. For example, a green ball cannot be heated too quickly on a hearth or it may explode. One manner of avoiding explosion of these green balls is to cool the hearth before introducing them thereon, this cooling step causing an extremely inefficient waste of heat. Thereafter, the hearth, carrying the green balls, must pass through a drying zone to gradually drive water from the green agglomerates, followed by rapid heating in a reduction zone to bring the agglomerates to a reducing temperature. This need for gradual heating causes thermal inefficiency of a rotary hearth furnace, and hearth cooling reduces hearth refractory life.
In the INMETCO process, it has been proposed that an alternative way to avoid green ball explosion upon rapid heat-up of and water expulsion from the green balls, is to use relatively large quantities of binder agents in the recipe used to make the green balls. This method, however, requires a binder dosage that can be up to 500% higher than the binder dosage required if a drier and relatively fine grinds of starting materials are employed in the process. In this case, the high binder dosage becomes an even more significant operating cost of the process. Overall, the binders required for these processes typically account for about 6 to about 15% of the total production costs for the direct reduction process.
The MAUMEE process, for certain starting materials, may be able to function without binders; however, the resulting briquettes are more fragile than even dry green balls. Therefore, this process requires special and more costly material handling equipment and practices. Furthermore, the MAUMEE process requires high capital costs due to the need for expensive briquetting equipment, and high operating costs due to wear of the molds used in the briquetting machines. All of these considerations, along with the need for intermediate bins and material handling equipment, are associated with large capital and operating costs in reduction processes using green balls, dry pellets, briquettes or other agglomerates as the feedstock. Therefore, there is a great need in the art for a direct reduction process which avoids the need for agglomeration of starting materials.
Another consideration which leads to overall inefficiency and other problems in carbonaceous direct reduction processes of the prior art, is that, due to the relatively low compression strength of wet and, to a lesser extent, dry green balls, it is not feasible to utilize a "surge bin" immediately ahead of the hearth. The term "surge bin," as used herein, refers to a container which holds material to be introduced onto the hearth prior to the introduction thereof. The surge bin may, for example, have coupled thereto a simple table feeder and a conveyor for delivering the feed material directly to the hearth feed system; and provides a buffering function, enabling the reduction process to continue even if one or more process steps for preparing feed material is interrupted due to, for example, equipment failure or planned maintenance. In the direct iron reduction processes known in the prior art, it is not feasible to employ such a surge bin because the wet green balls on the bottom of the bin will squash, deform and break due to the load of balls above them. Dry balls, although stronger than wet balls, will suffer a similar consequence, but to a lesser extent than wet balls.
In carbonaceous reduction processes which utilize green balls, therefore, it is not feasible to use a surge bin due to the physical characteristics of the green balls, and if the functionality of the balling unit is interrupted, the reduction process is halted. As such, the rotary hearth furnace becomes useless until the balling unit regains operability. The stops and starts of the rotary hearth furnace process result in the consumption of excess energy and also the production of off-specification or poor quality DRI. In processes which utilize dry agglomerates, a small amount of surge is possible (limited by the friable nature of the dry balls); however, any substantial interruption will cause this small amount of surge material to be consumed, and will, similarly, cause the reduction process to be disrupted with the attendant cost and quality penalties.
Halting of the reduction process, as occurs when the above situations arise, has extremely disadvantageous economic implications. Examples of these disadvantages include a decrease of annual output; an increased energy cost associated with thermal efficiency (it is costly to maintain the temperature of a rotary hearth furnace which is not in use, yet it is also very expensive to reheat the furnace prior to resuming the reduction process); and interruptions in the process are known to decrease the quality of the output material. Therefore, the inability to utilize surge bins is a substantial disadvantage of prior art processes with respect to annual output, process efficiency and product quality.
The present invention overcomes the aforementioned problems by teaching a process for achieving direct reduction of iron which avoids agglomeration steps and instead involves charging particulate starting materials onto a rotary hearth furnace. By utilizing inventive methods, the above-described problems associated with the formation of and reduction of agglomerates are avoided, along with the large capital and operating costs associated therewith.